Laser speckle (LS) imaging is a well known technique. A laser speckle image is obtained by imaging coherent light scattered from a target. If the target includes scatterers at a variety of different depths, the scattering will cause the originally coherent light to add constructively or destructively depending on the various path lengths from the various scatterers, resulting in an image with bright and dark spots, i.e. speckles. The speckle pattern will change if the scatterers are moving. Thus a laser speckle contrast image will tend to have lower contrast in areas where the scatterers are moving and smearing the speckle pattern. A laser speckle contrast image can be calculated based on one or more raw laser speckle images like this:
                              K          ⁡                      (                          x              ,              y                        )                          =                                                            σ                N                            ⁡                              [                                  I                  ⁡                                      (                                          i                      ,                      j                                        )                                                  ]                                                                    μ                N                            ⁡                              [                                  I                  ⁡                                      (                                          i                      ,                      j                                        )                                                  ]                                              ⁢                      ∀                                          (                                  i                  ,                  j                                )                            ∈              S                                                          (        1        )            where i indexes the rows of the image, j indexes the columns of the image, N is the number of raw laser speckle images being combined, I represents the intensity in an acquired raw laser speckle image, S represents either a single pixel (i, j) in several images across time or a limited region of the raw image around (i, j) in a single image such as a [5 pixel×5 pixel] window, σ represents the standard deviation of intensity I calculated either over S within a single image or at a single pixel over N images, μ represents the mean of the intensity I calculated either over S within a single frame or a single pixel over N images, and K(x,y) is the calculated laser speckle contrast at pixel (x,y) of the LS image.
The LS image can be used to calculate blood flow using the following equation:
                              K          2                =                                            τ              c                                      2              ⁢              T                                ⁢                      (                          1              -                              exp                ⁡                                  (                                                                                    -                        2                                            ⁢                      T                                                              τ                      c                                                        )                                                      )                                              (        2        )            where K is the LS contrast, T is the exposure time over which images are acquired, and 1/τc is an indicator of blood flow speed.